FIGS. 3a and 3b show an example of the structure of a conventional ferrule for an optical connector. In the figure, a ferrule 1 formed by molding thermosetting resin such as epoxy has a flat square shape and a front end surface 2 thereof is a flat surface. At the front end surface 2, a plurality of optical fiber insertion holes 3 are formed so as to be aligned in the horizontal direction, and pin engagement holes 4 are formed on the left and right sides of the optical fiber insertion holes 3 so as to sandwich the optical fiber insertion holes 3.
As shown in FIG. 3b, the optical fiber insertion holes 3 are formed so as to pass through the ferrule 1 from the rear end side to the front end surface 2. The front end sides of the optical fiber insertion holes 3 are fine holes so that optical fibers (bare optical fibers) are inserted therein without shaking. The openings of the optical fiber insertion holes 3 are rather wide in the vertical direction at the rear end side so that the coated portions of optical fiber cored lines may be inserted therein.
In the middle of each optical fiber insertion hole 3, an adhesive pouring hole 6 is formed. For ease of understanding, in FIG. 3b, the hole portion of the optical fiber insertion hole 3 through which a bare optical fiber is inserted is drawn so as to have a diameter greater than the actual diameter.
In manufacturing an optical connector by use of the ferrule 1, for example, an end portion of a multi-cored optical fiber line such as an optical fiber ribbon is stripped to remove the coated portion so that the bare fiber is exposed and under this condition, the multi-cored optical fiber line is inserted into the optical fiber insertion hole 3 from the rear end side of the ferrule 1 so that the bare fiber of each core is passed through the optical insertion hole 3 and protrudes out of the front end surface 2. Then, by pouring an adhesive into the adhesive pouring hole 6, the adhesive enters the optical fiber insertion hole 3, so that the optical fiber is secured by the adhesive in the optical fiber insertion hole 3. At this time, an adhesive 7 entering the optical fiber insertion hole 3 is hardened seeping out of the front end surface 2 of the ferrule 1 as shown in FIG. 4.
Under this condition, first, the front end surface 2 is ground so as to be flat, i.e. flat grinding of the front end surface 2 is performed, so that a bare optical fiber 8, an adhesive 7 and a portion A of the front end surface of the ferrule 1 shown in FIG. 4 are ground. Then, a portion B is obliquely ground, and the bare optical fiber 8 of the multi-cored optical fiber line is exposed out of a slanting surface 10. By the oblique grinding, the manufacture of an optical connector is completed.
In connecting optical connectors, a pin is inserted into the pin engagement hole 4 in the front end surface (connection end surface) of an optical connector and by inserting the ferrule of another optical connector from the other end side of the pin, the optical connectors to be connected to each other are aligned. Thereby, the optical fiber of one optical connector and the corresponding optical fiber of the other optical connector are brought in contact with each other so as to be opposed to each other with their axes coinciding with each other. The connector connection of the optical fibers is thus achieved.
The oblique grinding of the ferrule 1 is performed in order to reduce the connection loss by preventing the influence of reflected light of the connected end surface of the optical fiber by forming to be slanting the surface of the bare optical fiber 8 exposed out of the connected end surface.
However, in the conventional ferrule 1, since the front end surface is a flat surface vertical to the central axes of the optical fiber insertion holes 3, it is always necessary to perform the flat grinding for surely removing the portion of the adhesive 7 hardened seeping out of the front end surface 2. In addition, since it is necessary to perform the oblique grinding after the flat grinding, the number of grinding steps increases and since both the flat grinding and the oblique grinding are performed, the amount of grinding increases, so that the time required for grinding increases. This stands in the way of increasing the work efficiency of the grinding.
While the pin insertion holes 4 and the optical fiber insertion holes 3 are designed so as to be parallel to one another, in actually forming them, these holes tend to incline. Even when there is an inclination of the holes, in the molded products, the relative position distance between the pin engagement holes 4 and the optical fiber insertion holes 3 are accurately positioned on the side of the front end surface 2 of the ferrule 1. However, the amount of deviation between the relative positions of the holes increases from the front end surface 2 to the inside of the ferrule 1.
In grinding the conventional ferrule 1, as shown in FIG. 4, on the central axis of the bare optical fiber 8, in the first flat grinding, the front end surface 2 of the ferrule 1 is ground away by a thickness d.sub.A, and in the subsequently-performed oblique grinding, the front end surface 2 is further ground away by a thickness d.sub.B, so that on the central axis of the bare optical fiber 8, the front end surface 2 of the ferrule 1 is ground away by as much as a thickness of D.sub.0 (D.sub.0 =d.sub.A +d.sub.B) as a whole. For this reason, when the optical fiber insertion holes 3 slightly incline and when the pin engagement holes 4 slightly incline, the amount of deviation between the positions of the holes 3 and 4 increases as the amount of grinding of the ferrule 1 in the direction of the thickness increases, so that the connection characteristics of the optical connectors degrade.
The present invention is intended for solving the above-mentioned themes, and an object thereof is to provide a ferrule for an optical connector and a method for manufacturing an optical connector by use of the ferrule wherein the number of grinding steps and the amount of grinding of the ferrule are decreased, the work efficiency of the manufacture is increased and an optical connector being excellent in connection characteristics is obtained.